Rain detection by means of image processing of images taken by a camera which is focused on a windshield is known. Such rain detection can be realized by looking for droplets on the windshield. A disadvantage of such rain detection is that the camera needs to be focused on the windshield, whereas cameras used as sensor for advanced driver assistance systems in vehicles are used to monitor the surroundings of the vehicle and are therefore focused to infinity or almost to infinity.
PCT publication No. WO 2006/024247 A1 discloses a method for detecting precipitation, especially rain and fog, on a windscreen, which is not based on an active illumination of the windscreen or a measurement of the reflectivity, but on a camera using a plurality of adjacent pixels for detecting an image of a target area. Said target area is located in the surroundings of the vehicle, and the windscreen is thus represented in a blurred manner. In the event of a blurred representation, raindrops or fog on the windscreen produce a soft-focus effect in the image. The sharpness of the image and/or the difference in contrast of adjacent pixels is evaluated and the presence of precipitation is detected therefrom, the image is then preferably subjected to a two-dimensional Fourier transformation and the spatial frequencies are evaluated. A first image is recorded especially for a low contrast target area, a windscreen wiper passing over the windscreen in the visual region of the camera or a heating process is activated, and a second image is then recorded, and both images are evaluated according to changes.
U.S. Pat. No. 7,860,275 B2 discloses a method for detecting raindrops on a windscreen, which uses a camera, which is fitted inside the vehicle opposite the windscreen and which is focused at infinite. The method comprises steps of acquiring images by a camera through the windscreen, camera preferably focused at infinite, retrieving the contours of the spots present on the acquired images, and realizing the histogram of the widths of contours of the spots. It has been proven that a spot corresponding to a droplet of water on the windscreen could be distinguished from other spots (corresponding for example to an object of the landscape outside the vehicle) due to its width of contour, which may be quantified by a certain number of pixels. The droplets of water on a windscreen indeed present a contour, which is relatively wider than other spots. Establishing a histogram of these widths of contour makes it possible to select the contour points corresponding to the droplets, then eventually “to reconstruct” these droplets in the images by processing, in order to characterize them finally.
German patent publication No. DE 102011055343 A1 discloses a method for the detection of rain from an image of a camera that is partially focused on the windscreen of a vehicle. The method involves detecting an edge in an evaluation area of the camera image, and determining a measurement of the increase of the edge. The edge with a steep rise is classified as part of the potential raindrop on the disk imaged in the evaluation area. The span width and standard deviation of intensity values (X-1 to X-9) inside a window are evaluated with adjacent pixels for determination of the increase of edge at a pixel position (p-11 to p-45), where the pixel position lies in the center of the window.
Tong, H., Li, M., Zhang, H., & Zhang, C. (Tong et al.) propose a method for “Blur detection for digital images using wavelet transform” in their paper of the same title presented at Multimedia and Expo, 2004, ICME '04, 2004 IEEE International Conference on (Vol. 1, pp. 17-20), IEEE.
Nashashibi et al. present in their publication “Detection of Unfocused Raindrops on a Windscreen using Low Level Image Processing”, International Conference on Control, Automation, Robotics and Vision: ICARV '2010, December 2010, Singapore, pp. 1410-1415, a method where the camera is focused at infinity and rain is detected via image processing. The detection of rain in this publication is based on local intensity of the pixels and number of edges change when there is a drop on the windshield.
Several disadvantages of this method are mentioned in the subchapter B. (Discussion) of the chapter V. (Experimental Results) of the publication.
As such, it is desirable to present a method to determine the quality of an image by detecting heavy rain on a windshield of a vehicle with a camera. In addition, other desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.